Conventionally, in an inverter circuit for driving a motor, an insulated gate bipolar transistor (IGBT) has been widely used as a switching element. The IGBT has the characteristic in which an on-resistance is lowered by conductivity modulation of injected electrons and holes and accordingly has an advantage of a small loss in a high voltage/high current operation. However, in the IGBT, since both of electrons and holes contribute to conduction, there are a problem that the IGBT is unsuitable for fast switching due to a tail current generated when a switching element is turned off, and another problem of a large loss in a low voltage/low current operation.
In recent years, in equipment such as an air conditioner with a motor as a load, in order to increase an annual performance factor (APF), there has been a strong demand for reduction in loss which occurs not only in a high power load operation such as a start-up or the like but also in a low power load operation in the stationary conditions. For the purpose of meeting such a demand, it has been examined that a metal oxide semiconductor field effect transistor (MOSFET) having a smaller loss in low voltage/constant current than the IGBT is used as a switching element. In particular, an MOSFET having a super junction structure (hereinafter sometimes referred to as an “SJ-MOSFET”) can realize a higher breakdown voltage (e.g., a drain-source voltage of 500 volts or higher) with a lower on-resistance than the existing MOSFET, as will be described later. In addition, the SJ-MOSFET is suitable for high frequency switching since it is a unipolar device. Therefore, the SJ-MOSFET is expected to replace a part of an IGBT in an inverter.
However, a conventional SJ-MOSFET has a problem of a large reverse recovery current (hereinafter denoted by “Irr”) occurring in an internal parasitic diode (body diode), which extends a reverse recovery time (hereinafter denoted by “Trr”). This problem causes the convention SJ-MOSFET to be used less in an inverter circuit for driving a motor of an air conditioner, in practice. This is because, in such an inverter for motors, for example, when a reflux diode of a lower arm side switching element constituting each leg (half bridge) of a bridge circuit flows a current in a forward direction in a reflux mode, there is a timing at which an upper arm side switching element is switched from a turn-off state to a turn-off state, and, at that timing, a reverse recovery current Irr occurs in the reflux diode and a short-circuit current flows through the upper and lower arms. When the reverse recovery current Irr is large, a switching loss becomes excessive due to such a short-circuit current. Therefore, in the conventional SJ-MOSFET, the body diode with the large reverse recovery current Irr cannot be used as the reflux diode and there is a need to connect a fast recovery diode (FRD) element functioning as the reflux diode in parallel between drain and source.
For the purpose of overcoming this problem, there has been proposed an SJ-MOSFET which is capable of significantly reducing a reverse recovery current Irr by forming a local trap level in a semiconductor and controlling lifetime of minor carriers.
However, in a case where such an SJ-MOSFET is used in an inverter for motors, when the SJ-MOSFET has a miniaturized structure, it has been found that there may occur a “self-turn-on” effect, i.e., “shooting-through,” in which the SJ-MOSFET, which should be originally in a turn-off state, is conducting with no intention.